Electrophotographic marking is a well-known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, referred to herein as the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner particles to produce a black toner image. A recharge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various color toner particles are placed in superimposed registration so that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
The REaD IOI process can be implemented in a various ways. For example, in a single pass printer wherein the composite image is produced in a single pass of the photoreceptor through the machine. Another implementation is in a four pass printer, wherein one color toner image is produced during each pass of the photoreceptor through the machine, and wherein the composite color image is transferred and fused during the fourth pass. REaD IOI can also be implemented in a five cycle printer, wherein only one color toner image is produced during each pass of the photoreceptor through the machine, but wherein the composite color image is transferred and fused during a fifth pass.
In electrophotographic printing the step of conveying toner onto a latent image is called development. In development, charged toner particles are applied to a latent image such that toner particles electrostatically adhere to the proper areas of the latent image. There are several types of developers. A magnetic brush developer uses a two-component developer material that is comprised of toner particles and magnetic carrier beads. The smaller toner particles triboelectrically adhere to the larger magnetic carrier beads. When the developer material is placed in a magnetic field, the carrier beads and their triboelectrically adhering toner particles form relatively long chains which resemble the fibers of a brush, thus the name magnetic brush developer. When the magnetic brush is introduced into a development zone adjacent the latent image the electrostatic charges of the latent image pull toner particles from the carrier beads and onto the photoreceptor. The magnetic fields are typically created using a "developer roll," a cylindrical sleeve that rotates around a fixed assembly of permanent magnets.
Another developer is the "scavengeless" developer. Scavengeless development is described more fully in, for example, U.S. Pat. No. 4,868,600 to Hays et al., U.S. Pat. No. 4,984,019 to Folkins, U.S. Pat. No. 5,010,367 to Hays, and U.S. Pat. No. 5,063,875 to Folkins et al., and in their citations. In scavengeless development toner particles are conveyed onto a latent image using AC electric fields that are applied to electrode structures, commonly wires, that are positioned between a toner-loaded donor roll and the photoreceptor. Significantly, there is no physical contact between the developer and the photoreceptor.
A variation of the scavengeless developer is the "hybrid" scavengeless developer. A hybrid scavengeless developer includes a developer housing with a toner reservoir, a developer roll, a donor roll, and an electrode structure. The developer roll operates like a developer roll in the magnetic developer, except that instead of conveying toner particles directly onto the photoreceptor the hybrid scavengeless developer roll conveys toner particles onto a donor roll that is disposed between the transport roll and the photoreceptor. The donor roll is electrically biased such that toner particles are attracted from the developer roll. When loaded with toner the donor roll can convey toner particles to the photoreceptor. To do so, the electrode structure is AC-biased relative to the donor roll. The AC bias detaches toner from the donor roll into a toner powder cloud that forms in the gap between the donor roll and the photoreceptor. The latent image then attracts toner particles from the powder cloud, developing the latent image. It should be noted that when the donor roll bias and AC bias are removed the toner on the donor roll tends to leave the donor roll and move toward the developer roll and the toner reservoir.
Hybrid scavengeless development is advantageous in REaD systems because the donor roll acts as an electrostatic "intermediate" between the photoreceptor and the developer roll. This tends to reduce unwanted interactions between the developer and the photoreceptor. While hybrid scavengeless development does reduce unwanted interactions, two specific REaD interactions can still occur: scavenging and direct image contamination. In scavenging, toner on the photoreceptor is pulled off the photoreceptor and into a developer. This causes contamination of the toner in the developer. In direct image contamination, toner within a developer is attracted onto the photoreceptor where it is not wanted.
Both scavenging and direct image contamination are products of potential differences between the developer and the photoreceptor. Consider a photoreceptor having a toner layer after the image area and its toner have been uniformly recharged to -500V. Assuming a donor roll at +100 volts, toner particles on the image area are attracted into the developer housing, resulting in scavenging. Now consider a photoreceptor having a developed toner layer that has not been recharged. The toner particles might be at about -200V. Assuming a donor roll at -400 volts, toner particles are attracted from the donor roll onto the toner layer, resulting in direct contamination (even if the AC bias is removed from the developer electrode). While it is possible to set the donor roll bias at a potential that tends to balance scavenging and direct contamination, some scavenging and some direct contamination will then occur at all times.
In some REaD IOI systems a developer's donor roll is loaded with toner just before that developer develops its toner image. After development the toner on the donor roll is then returned to the toner reservoir. Since direct contamination is only a problem when toner is on the donor roll, direct contamination will not occur until the donor roll is loaded with toner.
In view of the detrimental effects of scavenging and direct contamination, and in view of their competing natures, a technique of reducing both would be beneficial.